This invention relates to an apparatus for calculating vehicle-wheel revolution values for detecting parameters such as the number of vehicle-wheel revolutions and the speed of vehicle wheels, calculating parameters of vehicle-wheel speed and acceleration from the detected parameters, and obtaining estimated values of the vehicle-wheel revolution values from these parameters.
The related-art includes an apparatus which obtains vehicle-wheel revolution values such as the number of vehicle-wheel revolutions, vehicle-wheel revolution speed and acceleration, and which obtains vehicle-wheel speed responsive to vehicle speed, has a device that removes noise from the signals detected by sensors. The noise includes not only electric noise but also noise caused by the movement of the vehicle wheels.
Vehicles are provided with a vertically movable suspension so that vehicle wheels W can absorb vibrations resulting from a rough road surface. Vehicles are also provided with a caster angle .theta. to move forward smoothly and to obtain antidive and antilift control. The vehicle wheels W move relative to the vehicle based on the suspension structure and the caster angle .theta.. When the vehicle wheels W vertically displace by a distance .DELTA.H relative to the vehicle, they also longitudinally displace by a distance .DELTA.L. As a result, even when the vehicle runs at a constant speed, the revolution speeds of the vehicle wheels W change due to the longitudinal displacement distance .DELTA.L of the vehicle wheels W, when the vehicle wheels W vertically move on a rough road surface. Accelerations G calculated based on the vehicle-wheel revolution speeds also change remarkably. When the vehicle wheels are eccentric, the vehicle wheels rotate unevenly, and noise generates on the vehicle-wheel speed and acceleration values calculated responsive to the revolution of the vehicle wheels.
The related-art apparatus for calculating vehicle-wheel revolution values removes the noise using a; moving-average type filter, a weighted-average type filter, or a low-pass filter. For example, when the vehicle-wheel accelerations are obtained using the weighted-average type filter, averages of instantaneous vehicle-wheel accelerations A1, A2...An detected at a sampling time .DELTA.T are obtained as follows: ##EQU1##
When noise is removed from the detected discrete values of vehicle-wheel accelerations An using the low-pass filter, the calculations are repeated using an accumulated value Bn as an intermediate buffer according to formula (2), thus updating estimated accelerations Dn. EQU Bn=K0.multidot.Bn-1+(An-Dn-1) EQU Dn=Dn-1+K2.multidot.Bn (2)
The calculated vehicle-wheel acceleration estimates are used for an antiskid control unit that prevents the vehicle wheels from skidding during braking (see Japanese Published Unexamined Patent Application No. S60-22548, titled "Antiskid Control Unit"), and for a traction control unit that prevents the vehicle wheels from slipping during quick starting or quick accelerating.
However, the related-art apparatus for calculating the vehicle-wheel revolutions takes averages of detected values or uses a low-pass filter with a time constant, resulting in the following problems:
(1) The parameters detected responsive to vehicle-wheel revolutions or the parameters to be calculated such as vehicle-wheel accelerations include accelerations caused by braking and detected for braking controls, and accelerations as noise due to vertical movement of the vehicle wheels on the rough road surface. If the detection signals are simply averaged using formula (1), the accelerations to be detected are averaged without being distinguished from the noise. As a result, the accelerations cannot be precisely detected. When sampling period is lengthened and the number n of acceleration data for taking averages is increased, too much time is consumed for detecting changes in accelerations, delaying the start of the control operations.
(2) When noise is removed using the low-pass filter and the coefficient K2 in formula (2) is reduced, then the amplitude of noise can be reduced, but the control operations are again delayed. In FIGS. 12A-12C, detection signals are shown in broken lines, having zero noise, noise half a base signal value and noise twice the base signal value. As shown in FIGS. 12A, 12B, and 12C, when the value of the coefficient K2 decreases, noise with amplitudes in some range can be removed from the detected signals shown by broken lines in FIGS. 12A, 12B, and 12C. However, the time T until the filtered value shown by solid lines in FIGS. 12A, 12B, and 12C intersects a threshold value Gth lengthens. As shown in the right graph of FIG. 12C, when noise has an amplitude out of the range, the time T lengthens markedly. The threshold value Gth indicates the level at which pressure starts decreasing under antiskid control.
(3) When the vehicle-wheel accelerations obtained by the related-art calculation apparatus having these problems are used for the antiskid control and other controls, the antiskid control unit or the traction control unit cannot respond quickly on rough road surfaces. The effectiveness of the antiskid control unit on rough road surfaces declines, and the braking distance lengthens as compared with that on a smooth road surface with less noise. If the value of the coefficient K2 is determined according to low-level noise, noise cannot be removed sufficiently, and the threshold value Gth should be adjusted to avoid malfunction due to noise. In addition, the control unit cannot respond quickly even on a smooth road surface. If the threshold value Gth is determined according to low-level noise, the antiskid control unit frequently adjusts the braking oil pressure, lengthening the braking distance.